1. Field of the Invention
The present invention relates to an optical gas concentration detector and a method of producing a structure used in the detector.
2. Description of Related Art
A gas molecule has a property of absorbing light having a predetermined wavelength, and this property is used in a variety of gas concentration detectors. Intensity of light absorbed by the gas molecule depends on an optical path length of light to be measured, and a gas concentration to be detected. Therefore, when the gas concentration is extremely low, the detector needs a long optical path length. Thus, a size of the gas concentration detector may become large.
JP-A-2005-147962 discloses a gas concentration detector. Light having the predetermined wavelength enters a gas cell of the gas concentration detector, in which a predetermined gas exists. A plane mirror disposed in the gas cell performs multiple reflections of the light, and the reflected light is received to detect the gas concentration. Thus, the optical path length can be increased by the multiple reflections. Therefore, the gas cell having a relatively small size can secure relatively long optical path length. Further, gas having relatively low concentration can sufficiently absorb light.
The above-described gas concentration detector needs a pair of mirrors, especially concave mirrors, capable of enclosing light in the gas cell. The concave mirror is made of a material, e.g., Si, Ge or ZnSe, making light to pass through, and the light has a wavelength range from mid-infrared to far-infrared in order to detect the gas concentration. However, when the concave mirror is made of this material, a surface of the concave mirror has to be grinded after machine processing. In this case, cost for producing the concave mirror may be increased. Therefore, the machine processing is required to be performed easily with high accuracy.
In contrast, US 2005/0133478 A1 (corresponding to JP-A-2005-181961) discloses a method of producing a concave mirror by using a silicon substrate (first layer). Specifically, a second layer is formed on the first layer. Etching rate is different between the first layer and the second layer. Then, a mask pattern is formed on the second layer. Thereafter, each layer is etched with each etching rate. Thus, lens can be formed on the silicon substrate.
However, laser diode (LD) or light-emitting diode (LED) having a high directivity is needed in JP-A-2005-147962, in order to perform the multiple reflections of light. In this case, quantum cascade laser light source may be used as a light source, because the quantum cascade laser light source can emit light having wavelength equal to or larger than 2 μm in the range from the mid-infrared to the far-infrared. When the quantum cascade laser light source is used, gas can have a high coefficient of absorbing light, so that the gas can be detected with a high sensitivity. However, the quantum cascade laser light source is expensive, so that it may not be practical to use the quantum cascade laser light source.
Therefore, an electric bulb using hot wire (heat ray) is generally used as the light source. The electric bulb can emit light having continuous wavelength, which depends on a temperature of the light source. However, the directivity of the electric bulb is low compared with that of the LD or LED. That is, a light beam in the gas cell disclosed in JP-A-2005-147962 is not stable, so that the optical path length cannot be secured to be stable.
Further, the difference in the etching rate between the first and second layers is used to produce the concave mirror in JP-A-2005-181961. However, unevenness (variation) may be generated in a shape of the concave mirror due to unevenness (variation) in a thickness of the second layer.
In addition, JP-A-2003-185803 discloses a method of forming a concave surface in a trench formed in a substrate by using isotropic etching. This method may be superior to the method disclosed in JP-A-2005-181961 in a point of controlling the shape of the concave mirror. However, in order to perform the multiple reflections, mirror has to have a size in a millimeter order, for example. The method disclosed in JP-A-2003-185803 can form a mirror having a size in a micrometer order, but cannot form a mirror having a size in the millimeter order.